History of the Tlieory of Heredity. 67 



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In all six cases the eggs gave rise to normal embryos; 

 bat that this was really due to the pre?ence of sperma- 

 tozoa in the water, was shown by the fact that no such 

 surprising result followed in a second set of experiments 

 where especial effort was made to get pure sea-water. 

 Many of the recorded cases are open to the same objec- 

 tion; and in other cases, as in the virgin sow referred to 

 by Bischoff, there seems to be some doubt whether the 

 ova were really undergoing development; but Oelacher's 

 observations on the eggs of a virgin hen ('^Die Veriln- 

 derungen des unbefruchteten Keimcs dcs Hiihnereies 

 imEileiter und bei Bebriitungsversuchen," Zeit. f. Wiss, 

 Zool., xxii., 1872, p. 220) seem to show that the hen's 

 egg does have the power to pass through the first stages 

 of development whether it is impregnated or not. 



The instances of parthenogenesis which I have given 

 show that this power may be independently acquired 

 by animals which cannot possibly inherit it from a com- 

 mon source. In the vast majority of insects, and in 

 the majority of the Crustacea, the egg does not show the 

 slightest tendency to develop before it is fertilized. It 

 is true that in the case of the Crustacea the evidence 

 for this statement is almost entirely of a negative char- 

 acter, for no one has ever shown by experiment on any 

 considerable number of species that the female cannot 

 lay fertile eggs when the access of a male is prevented, 

 but in many insects we know from actual observation 

 that the eggs die soon after they are laid, unless they 

 are fertilized ; and we know enough of the breeding 

 habits of Crustacea to feel confident that parthenogenesis 

 is exceptional among them, just as it is among insects. 



We must, therefore, conclude that if we could retrace 

 the course of evolution of any parthenogenetic animal 

 we should be led back to an ancestral form which never 



